Inhalable aerosols hazards are a significant contributor to the burden of occupational respiratory disease in this country. They are present in virtually every NORA sector: organic dusts in agriculture, forestry, and fishing; carbonaceous and mineral dusts in construction; metal and metalworking fluid aerosols in manufacturing; and coal dust in mining are but a few examples. However, we currently lack the technology to determine the size distribution of inhalable aerosol hazards in the workplace. Without this information, our ability to associate risk with exposure to inhalable aerosols is severely hampered. Therefore, the objective of this research is to develop new, inexpensive technology to characterize the size distribution of inhalable aerosol hazards in the workplace. Such a sampler will enable occupational health professionals to assess inhalable aerosol hazards more fully, as no commercially-available technologies currently exist to meet this need. The project has three specific aims: (1) Design an inexpensive, upflow clarifier to characterize the size distribution of inhalable aerosol hazards in the workplace (ranging from 10 to 100 5m); (2) Evaluate device performance using a combination of calm-air chamber studies and low-velocity wind tunnel experiments; (3) Test sampler performance in the field by characterizing inhalable particle size distributions for organic dust hazards in dairy and grain handling facilities. We will quantify inhalable organic dust size distributions as a function of mass and endotoxin content. The latter measure has never before been reported as a function of particle size for particles larger than 10 5m. This research contributes to the aims of the NORA by developing an exposure assessment technique capable of generating new and important knowledge for the field. Although inhalable aerosol samplers exist for measuring the total concentration of particles, we currently have no means for determining inhalable particle size distributions. Because size (in conjunction with chemical composition) plays a primary role in determining health effects, knowledge of the size-resolved chemical composition is of great interest to industrial hygienists, occupational physicians, and epidemiologists alike.